1. Field of the Invention
The invention relates to a recording or replay device for corrected-focus optical scanning when recording on or replaying information from an optical recording medium, containing a radiation source which provides a scanning beam, a focusing objective lens as well as a radiation-sensitive detector system which essentially lies on a plane, and at least one diffraction element for producing auxiliary beams for deriving a focus error signal.
2. Description of the Prior Art
It is known, when playing back from optical recording media, so-called compact disks or CDs, for digital information to be read with the aid of optoelectronic scanning system, projections or depressions, the so-called pits, which are arranged on the data medium being detected without any contact by means of a focused light beam from a light source, for example a semiconductor laser, when the optical recording medium is rotating at speeds from 200 to 500 revolutions per minute. The light beam which is produced by the radiation source is passed to the optical recording medium via an optical system which may have mirrors, lens and/or prisms. When the light beam strikes a pit, diffraction and/or interference phenomena occur in contrast to reflection on a plane or on the surface between the individual pits, and these phenomena lead to modulation of the reflected light beam. The light beam, modulated by the existing information pattern of the recording medium, is then passed to a detector system, for a example a photodiode, via a semireflective prism or the like. The detector system is used to convert the modulated light beam into electrical signals, from which downstream evaluation electronics are used to obtain the information which is present on the recording medium, for further processing. An electronic servo circuit ensures the tracking of the light beam in such a manner that said light beam follows the track of pits accurately, and in such a manner that, in spite of any unevenness of the recording medium which normally cannot be avoided, and any other mechanical or thermal disturbances, this light beam is focused with high precision onto the layer of the recording medium that carries the information.
EP-0 373 699 B1 discloses an optical scanning device which, inter alia, achieves the object of obtaining a focus error signal from the optical signal, so that the desired focus readjustment can be carried out with appropriate accuracy.
It is proposed for this purpose that a mirror objective be designed which has two opposite surfaces and is used to focus the radiation from a laser source onto the CD, the surface which faces the radiation source having a radiation window which lies around the optical axis of the mirror objective, and having a reflector which surrounds this window. The second surface, which faces away from the radiation source, has a reflector which lies symmetrically around the optical axis, and has a radiation window which surrounds this reflector.
The first radiation window is fitted with a first diffraction element, for example a grating, in order to obtain a scanning beam and two auxiliary beams from the beam supplied by the radiation source. The second reflector is fitted with a second diffraction element, in order that part of the radiation which is reflected by the object to be scanned and passes through the mirror objective once again can be deflected in the direction of a radiation-sensitive detection system. This part of the scanning beam is deformed by the diffraction element such that a focus error signal FES can be derived with the aid of the detection system. The auxiliary beams, which are received by the first diffraction element, allow track control to be carried out, that is to say track control on the basis of a track error signal TES.
The focus error signal obtained with the aid of the second diffraction element and the detection system indicates the magnitude and the direction of any error in the instantaneous focus plane with respect to the mirror objective, it being possible to use this error signal for tracking the laser radiation, for example by means of a focusing coil.
By using a diffraction element having two grating elements, which split the light beam into two beam elements, and using two detector pairs, it is possible by determining the change in the position of focus spots on the detectors to check the extent of any focus error of the light beam with respect to the layer which carries the information. The output signals of the detectors are compared to obtain a parameter for assessing the focus error which, as described, can be used for regulation.
The grating used for the method according to EP 0 373 699 B1 described above is a diffraction grating having two grating elements, which split the diffracted beam into the said beam elements, the detection system comprising detector pairs, and a first and a second beam element, respectively, interacting with a first and a second detector pair, respectively. The grating elements, which touch at one edge, act like an edge which is used in the so-called Foucault method, that is also called a knife edge, the rows of focus spots produced by the gratings in each case lying in the same focus plane, and the error between the focus spot and a predetermined point or a predetermined line representing the size of the focus error.
As a result of the fact that the servo light beam is focused onto the detector system in the Foucault method, the focus error signal obtained has only an inadequate signal-to-noise ratio, so that relatively sensitive detectors have to be used or the complexity of a downstream electronic circuit is increased. Furthermore, the focus error signal based on the Foucault method is sensitive to position changes between the optical system and the detectors, which results in an increased error rate.
DE 44 42 976 A1 discloses a device for non-touching optical scanning of information, the basis of this once again being a light source which transmits light to one of the recording tracks and has a detector arrangement which receives light reflected from the recording track. A beam splitter and an objective lens are located in the beam path between the light source and the detector arrangement. In addition, arranged between the light source and the beam splitter there is a micro-mirror whose mirror surface can be deformed and/or swivelled by means of a drive, by which means it is possible to vary the incidence angle and/or the incidence point of the light beam passing from the light source, via the micro-mirror to the beam splitter, the objective lens and the recording track, as a function of the swivel position and/or deformation of the micro-mirror. This is intended to enable quicker and more accurate track finding and tracking with less complexity. According to DE 44 42 976 A1, the beam splitter may be formed by a holographic element which, on one side of a glass substrate, has a hologram for splitting the light beam reflected from the recording track into beam elements. The hologram is split into a first region with a narrow grating structure and into a second region with a broader grating structure, by which means the beam reflected from the recording track is split into the said beam elements, which strike different area regions of a detector arrangement, in order that, in this way, it is possible to evaluate them both independently for obtaining signals for tracking and for focusing. The beam element, which is produced by small hologram grating spacings, is imaged on a dividing line between two detection areas. The beam element of the main beam produced by large hologram grating spacings is directed at a further detection area. The beam element produced by the small grating spacings, which is passed to two adjacent and opposite detection areas, is used to derive the focus error signal for controlling the drive for the optics by detecting the respective spot light intensities. The beam elements are thus split and imaged onto in each case two detection areas, in order use the intensity difference between the two areas to deduce the focus error. The disadvantages of the solution according to DE 44 42 976 A1 are as follows.
As a result of the beam elements being imaged in one focus plane, the intensity of the focus spots is low so that, particularly when the scanning beam is in a focused position, the signal obtained from detectors has an unfavorable signal-to-noise ratio.
The object of the invention is thus to specify a device for corrected-focus optical scanning or recording of information from or, respectively onto a recording medium, which device uses a simple optical scanning system to allow it to supply focus error signals with a favorable signal-to-noise ratio and which allows it to operate with a less sensitive detector system, as a result of which the costs for implementation of such a scanning device are reduced, in particular by the adjustment complexity being reduced.
The object of the invention is achieved by an object according to the features of Patent claim 1, advantageous refinements and developments being specified in the subclaims.